The keto diet has blown up as an ultra-low carb eating plan that can help you drop pounds fast—but its effects on your body go beyond weight loss.

A typical keto diet is comprised of 80% fat, 15% protein, and a mere 5% of calories from carbohydrates. If you consume 2,000 calories a day, that means just 100 of them are coming from carbs—including healthy carbs like fruits and vegetables. When you eat this way, it triggers ketosis, which means your body has burned through all its carbs and needs to begin burning fat for energy.

It’s true: Following a strict high-fat, low-carb regimen can help move the number on the scale, but there might be some other keto diet side effects that you aren’t aware of. Some of them are positives, but a few could be unpleasant—or even dangerous. Here’s what you should know about keto diet dangers before you decide to try it for yourself.

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You might get hit with the “keto flu.”

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Keto flu is a real thing. Cutting your carbs to the bone and going into a state of ketosis (where your body burns fat for energy) can bring on a cluster of uncomfortable symptoms, such as headaches, fatigue, muscle aches, nausea, and diarrhea. The side effects are the result of your body transitioning to using fat as its primary source of energy instead of carbs, explains Kristen Mancinelli, M.S., R.D.N., author of The Ketogenic Diet. Once it adapts to the new fuel source (usually within a week or two), you’ll start to feel better.

The keto diet is notorious for delivering a quick initial slim down. That’s because carbs hold on to more water than protein or fat, says Becky Kerkenbush, R.D., a clinical dietitian at Watertown Regional Medical Center. So when you stop eating them, all that extra H2O gets released through urination. As a result, the scale might read a few pounds lower, and you may look a bit leaner.

That first drop might be mostly water weight. But research suggests that the keto diet is good for fat loss, too. An Italian study of nearly 20,000 obese adults found that participants who ate keto lost around 12 pounds in 25 days. However, there aren’t many studies looking at whether the pounds will stay off long-term, researchers note. Most people find it tough to stick with such a strict eating plan, and if you veer off your diet, the pounds can easily pile back on.

Constipation could be just around the corner.

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Constipation is a common side effect of low-carb eating plans, including the ketogenic diet. Severely curbing your carb intake means saying goodbye to high-fiber foods like whole grains, beans, and a large proportion of fruits and vegetables, says Ginger Hultin, M.S., R.D.N., Seattle-based nutritionist and spokesperson for the Academy of Nutrition and Dietetics.

Combine that with the fact that your body is excreting more water, and you have a potential recipe for clogged pipes. You can keep things moving by getting some fiber from keto-friendly foods like avocado, nuts, and limited portions of non-starchy vegetables and berries, says David Nico, Ph.D., author of Diet Diagnosis. Upping your water intake helps, too.

There’s also a side effect called “keto breath.”

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When your body goes into ketosis, it will start to produce by-products called ketones. This includes acetone—yes, the same chemical found in nail polish remover, which your body actually naturally makes on its own, according to a 2015 review of research. Because acetone is a smaller molecule, it tends to make its way into your lungs. You’ll eventually exhale them out, resulting in “keto breath.” Your mouth might also have a metallic taste, but it won’t last forever as you adjust to the diet. Just be diligent about brushing your teeth!

You’ll probably be thirsty all the time.

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Don’t be surprised if you find yourself parched while you’re on the keto diet. Excreting all that extra water will likely cause a spike in thirst—so make it a point to drink up, Mancinelli advises. There’s no hard and fast recommendation for how much water you should be having on a keto diet. But in general, aim to drink enough so your urine is clear or pale yellow. If it’s any darker, bump your intake.

...but your appetite won’t be as ravenous.

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Weight loss often means feeling hungrier and fighting off more cravings, but that doesn’t always seem to be the case when you go keto. People report less hunger and a diminished desire to eat after adopting a ketogenic diet, according to an analysis of 26 studies. Experts don’t fully understand why, but it’s thought that very low carb diets could suppress the production of hunger hormones like ghrelin.

And your skin might clear up!

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Plagued by pimples? You may start to notice a difference in your skin on the keto diet, especially if you were a former sugar addict. Consuming lots of empty carbs is linked to worse acne—in part because these foods trigger inflammation and signal the release of hormones that up the production of pore-clogging oils, according to a review published in the Journal of the Academy of Nutrition and Dietetics. Some findings suggest that curbing your carb intake could help solve these problems, improving your skin as a result.

Plus, many say they feel less brain fog.

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It’s no secret that carbs—especially refined ones like sugary cereals, white bread and pasta, or sweet drinks—cause your blood sugar to spike and dip. So it makes sense that eating less of them can help keep things nice and even. For healthy people, this can translate to more steady energy, less brain fog, and fewer sugary cravings, Mancinelli explains.

Your A1C levels could even improve.

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If you have diabetes, better blood sugar control could help lower your A1C levels—the measurement of glucose in our blood—and even reduce the need for insulin, according to a scholarly review of ketogenic diets. (Just don’t go off your meds without speaking to your doctor first!)

The one important caveat: Eating keto also ups the risk for diabetic ketoacidosis, a life-threatening condition where fat gets broken down too fast and causes the blood to become acidic. It’s much more common in people with type 1 diabetes, but if you have type two and are eating keto, talk with your doctor about what you should be doing to diminish your risk.

But your kidneys might get stressed.

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The kidneys play an important role in metabolizing protein, and it’s possible that eating too much of the nutrient can have a negative impact on kidney function. While ketogenic diets are supposed to be much higher in fat than they are in protein, many keto eaters make the mistake of loading up on lots of meat, Mancinelli says. The result? You could end up eating way more protein than you actually need.

Here’s the tricky part: There’s no definite answer for how much protein you’d have to eat before you run into trouble. “It really depends on how much protein a person is consuming versus how much they need, as well as the health of their kidneys at baseline,” Hultin says. That’s why it can be helpful to speak with a nutritionist or doctor who can help you tailor your diet before going keto.

And your heart disease risk factors could change.

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Eating an ultra-low carb diet is linked to a lower rate of obesity and type 2 diabetes, along with improved HDL cholesterol, all of which can translate to a lower risk for heart disease.

But your heart health might depend on what you actually eat. Research published in the New England Journal of Medicine suggests that low-carb diets based mostly on plant sources of fat and protein (like avocados or nuts) can lower heart disease risk by 30%. But those benefits didn’t hold for people who ate mostly animal-based proteins and fats. (Think: bacon, butter, and steak.)

Plus, the American Heart Association says that going overboard on saturated fat—which can be easy to do on a keto diet if you eat a lot of meat, butter, and cheese—can up your risk for heart problems. While you’re on the keto diet, you should have your cholesterol levels and heart health assessed by a doctor on a regular basis, Hultin says.

The bottom line?

Eating a keto diet can have some short-term health perks. But in the long run, it also has the potential to create some serious health problems. That’s why many experts say you shouldn’t attempt it on your own. “In general, if a person follows a ketogenic diet, they should only do so for a brief time and under close medical supervision,” says Hultin.

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